In a voice over Internet Protocol (VOIP) network infrastructure, traditional POTS (Plain Old Telephone Service) phones are replaced by new digital phones, called “IP phones,” which connect to a packet network (cabled or wireless) with protocols similar to a conventional personal computer (PC) or other packet network-compatible computing device. IP phones have different protocols of operation than traditional phones ,and standards have not yet been developed for specialized uses, such as the transmission of real-time text communication in teletypewriter (TTY) mode for IP phones.
A TTY (teletypewriter) phone, also called text telephones, are devices that are designed to transmit and receive text messages over the public switched telephone network (PSTN.) They permit a person with a hearing or speech impairment to communicate in real-time over the PSTN. A deaf scientist, Robert Weitbrecht, is credited with the development of the TTY in the 1960s. A teletypewriter is an input device that allows alphanumeric character to be entered and transmitted in real-time to a remote TTY device or printer. The Teletype Corporation developed the teletypewriter, which was an early interface to computers. Teletype mode is the capability of a computing device to handle teletypewriter input and output. TTY can transmit a single character or lines of characters in real-time.
Modern TTY phones generally comprise a keyboard, similar to that of a typewriter, and a text display. Older TTY phones might use a printing device to display incoming text rather than a display. The TTY may have a cradle to couple to a standard telephone handset, but more likely it has a direct connection to the PSTN via a standard RJ11 port.
A TTY user typically initiates a call by dialing the destination phone number. When a connection is achieved, the user types his message on the TTY keyboard. The text characters are immediately encoded using a voiceband encoding scheme and transmitted over the PSTN to the destination device. If the destination device is another TTY, that TTY decodes the text message and displays it for the recipient. If the destination device is not another TTY device, the user requires assistance from a third party, such as an operator. In such a situation, the TTY user must dial the third party, and request that person dial the destination device. The TTY user must transmit the message to the third party, who then relays the message to the final recipient by reading it aloud. The encoded voice signal of the intermediary is subsequently transmitted over the PSTN to the intended recipient. Similarly, in order to receive an incoming call from a standard telephone user, the TTY user must rely on a third party to intercept the voice message and translate onto a TTY device.
Each time a user types a key on the TTY, the information is encoded for transmission over the PSTN. The exact encoding algorithm can vary from TTY device to TTY device, with at least six known legacy algorithms in operation in various countries. The legacy algorithms are often incompatible with each other, which further limits a TTY user's ability to communicate. The six existing legacy algorithms are Baudot, EDT, DTMF, V.21, Minitel, and Bell103. In order to mitigate the legacy interoperability problem, the International Telecommunications Union (ITU) has advanced a newer standard, known as ITU-T V.18. The V.18 standard includes procedures for automoding, interworking, modulation and protocol conversion so that various legacy-mode TTY's may be able to communicate with a V.18 TTY. The V.18 standard also permits simultaneous voice and text communication. All future TTY's are expected to migrate to the V.18 standard.
In addition to the PSTN, voice communication can occur over packet networks, such as the Internet, using an IP phone or IP phone emulation software on a personal computer. The protocols for such communication is referred to as Voice Over Internet Protocol (VOIP). In a VOIP call, the device at either end of the communication link may be either a traditional telephone or a VOIP phone. An example of networks and components for a VoIP call is illustrated in FIG. 1. Access network 10 could be any network accessing the Internet such as an IP, Asynchronous Transfer Mode (ATM), or Ethernet network, which is a managed broadband network. Network 10 comprises a router 14 connected to various customer premise equipment and to media gateway 12. Media gateway 12 must be capable of detecting changing resource or network conditions. The ability to detect and monitor changing resource and network conditions can result in significant cost reductions and/or improved quality. Router 14 is connected to Internet Access Device (IAD) 16, wireless access point (AP) 22, and/or IP PBX (personal branch exchange) 23. A voice call may be placed between any of the customer equipment phones 18 connected to IAD 16, wireless IP phone 24 connected to AP 22, or IP PBX phone 30. Using special software, calls could also be placed through computer 20 connected to IAD 16 or portable computer 26 connected to AP 22.
Customer equipment is connected through access broadband network 10 to the Internet 34 by media gateway 12. On the far end is the PSTN 48, networking to POTS phone 52 through a Central Office 50. PSTN is also connected to the Internet 34 through a trunk gateway, composed of signal gateway 46, media gateway controller/proxy (MGC) 32, and trunk media gateway (MG) 42. IP and packet data 28 (e.g., real time protocol (RTP packet data)) associated with the call is routed between IAD 16 and trunk MG 42. The trunk gateway system provides real-time two-way communications interfaces between the IP network (e.g., the Internet) and the PSTN 48. Calls could also be placed between IP phone 40 through AP 38 and media gateway 36 and any of the customer phones through router 14.
However, when the transmission medium is a call between people with speech and hearing impairments is an IP network, or when a traditional TTY phone is connected to a VOIP gateway in order to receive IP calls, the signals can suffer from many impairments including incompatibility of TTY legacy protocols with IP network and IP phone protocols.